Yeah, you could technically calculate it directly in Replo, but would need a custom liquid snippet that divides the variant price by the number of KG per variant, and outputs it as €/kg text. Presuming metafield will work here though
Hmm haven't seen that before, usually if there are multiple variants it lets you select the default variant. If you mention it in support chat they should be able to advise as they will have access to your editor to see how it is set up